The compressor section of a turbocharger generates high frequency sound related to the blade pass frequency of the compressor. The sound is generally in the range of 6 kHz-12 kHz. In the past, this sound was not very noticeable. More recently, however, as other sources of noise in a vehicle have been reduced, compressor noise has become more of an issue.
One conventional system for addressing the noise issue identified above involves the inclusion of a discrete, duct-based component, for example a lined duct, which may be referred to as a duct silencer. Such a component has a foam lining on the outer wall and has diametrical partitions dividing the duct cross-sectional area into multiple sections. This system can be effective at the aforementioned high frequencies, but the inner diameter of the duct partitions of the silencer must be on the order of the sound wavelength (approximately one inch in diameter). Because the inlet and outlet duct diameter of an air induction system is typically several times this size, partitions or other features are required to achieve the required smaller effective diameter. The partitions are fairly thick (e.g., about ⅜″) because they contain numerous blind-hole, quarter wavelength resonators. The reduction in an overall cross-sectional area, combined with the circumferential ribs used to contain the foam, results in very high pressure drop at high air flow velocities, thereby reducing overall engine performance.
Another conventional arrangement for addressing the noise issue involves covering portions of the inside of the air filter housing. The portions of the air filter housing may be covered with a sound dampening or damping foam or other material. It has been observed that such an arrangement is not very effective unless large amounts of sound absorbing materials are used. However, using such large amounts of material becomes prohibitive from a packaging standpoint.